|Magnetic Vortex Core Reversal by Low-Field Excitations|
In micrometer-sized magnetic thin films, the magnetization typically adopts an in-plane, circular configuration known as a magnetic vortex. At the vortex core, the magnetization turns sharply out of the plane, pointing either up or down. Magnetic data storage based on this binary phenomenon is an intriguing concept, but it would require the ability to flip the vortex cores on demand. Because these structures are highly stable, very strong magnetic fields of around half a tesla (approximately one-third the field of the strongest permanent magnet) were previously thought to be necessary to accomplish this. At the ALS, a team of researchers from Germany, Belgium, and the U.S. has used time-resolved scanning transmission x-ray microscopy (STXM) to observe vortex motion and demonstrate the feasibility of using weak magnetic fields as low as 1.5 millitesla (mT) to reverse the direction of a vortex core. The observed switching mechanism, which can be understood within the framework of micromagnetic theory, gives insights into basic magnetization dynamics and their possible application to data storage technologies.
In magnetic thin films, magnetostatic interactions usually force the magnetization to lie parallel to the film plane. When further constrained to an area of about a square micrometer or less, the magnetic moments will form rotationally symmetric patterns that follow closed flux lines. At the center, the tightly wound magnetization cannot lie flat, because the short-range exchange interaction favors a parallel alignment of neighboring magnetic moments. The direction of the out-of-plane component is defined as the polarization of the vortex core. Moreover, the vortex structure can be set into gyrotropic motion by the application of a small magnetic field, and the sense of the gyration (clockwise or counterclockwise) is determined by the vortex core polarization. Thus, a change in the sense of gyration unambiguously indicates a change in the vortex core polarization.
In this experiment, the researchers applied a small (0.1-mT) sinusoidal magnetic field to induce gyrotropic motion in a square Permalloy (Ni80Fe20) sample. The excitation frequency was set at 250 MHz, close to the resonance frequency of the system (about 244 MHz) derived from micromagnetic simulations. The excitation field was synchronized with flashes of circularly polarized x rays from ALS Beamline 11.0.2 to produce dynamic STXM images, with x-ray magnetic circular dichroism (XMCD) providing the contrast mechanism. A short (4-ns) burst of 1.5 mT was applied, superimposed on the weak alternating field. The results show that the vortex core follows an elliptical trajectory, and a change in the sense of the gyration (and thus the vortex core polarization) can be clearly seen after the burst.
This vortex core switching, observed experimentally for the first time, was also reproduced by micromagnetic simulations. The simulations show that the burst distorts the out-of-plane vortex structure and creates a region of opposite out-of-plane magnetization at the edge of the original vortex core. This opposite magnetization grows and eventually splits into a vortex–antivortex pair with equal polarizations. The newly formed vortex and antivortex move apart, and the antivortex moves toward the original vortex. When the antivortex meets the original vortex, they annihilate each other, emitting spin waves in the process, until only one vortex, having a reversed polarization, remains in the structure.
These results show that properly tuned bursts of only 4 ns can be used to switch the polarization of a vortex core. Because the resonance frequency of the gyrotropic mode scales inversely with the lateral dimensions, much shorter pulses should be sufficient for switching the core polarization of smaller elements. Although their practical realization is still far off, data storage systems based on this core-switching scheme could have several advantages, including high thermal stability, insensitivity to external static fields, and minimal crosstalk between neighboring patterns, all of which are indispensable features for ultrahigh-density magnetic storage devices.
Research conducted by B. Van Waeyenberge (Max Planck Institute for Metals Research and Ghent University, Belgium); A. Puzic, H. Stoll, K.W. Chou, M. Fähnle, and G. Schütz (Max Planck Institute for Metals Research); T. Tyliszczak (ALS); R. Hertel (Research Centre Jülich, Germany); H. Brückl, K. Rott, and G. Reiss (Bielefeld University, Germany); and I. Neudecker, D. Weiss, and C.H. Back (University of Regensburg, Germany).
Research funding: German Research Foundation. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences (BES).
Publication about this research: B. Van Waeyenberge, A. Puzic, H. Stoll, K.W. Chou, T. Tyliszczak, R. Hertel, M. Fähnle, H. Brückl, K. Rott, G. Reiss, I. Neudecker, D. Weiss, C.H. Back, and G. Schütz, "Magnetic vortex core reversal by excitation with short bursts of an alternating field," Nature 444, 461 (2006).